In recent years, as the demand for indoor voice communication and data communication has grown due to the spread of mobile phones, the development of home-use base stations installed indoors has been pursued. As a method of operation of such a home-use base station, a way of implementing communication in which only a pre-registered mobile phone(s) is connected to a home-use base station has been studied in the third generation mobile communication system standardization project called “3GPP (3rd Generation Partnership Project)” (see Non-patent literature 1).
Since a area covered by a home-use base station is considerably smaller than that of a base station installed outdoors, the area is called “home cell”. Accordingly, a home-use base station is referred to as “home base station” hereinafter. Further, to distinguish an intended home base station from macro base stations in existing mobile communication networks that are installed in the vicinity of the intended home base station as well as other nearby home base stations, those nearby macro base stations and nearby home base stations are referred to as “adjacent base stations”. Further, the areas covered by those adjacent base stations are referred to as “adjacent cells”.
The home base station like the one described above has been studied for use in systems such as W-CDMA (Wideband Code Division Multiple Access). Between a home base station and a mobile station, data transmission is performed by using a dedicated channel, of which transmission power is controlled, on the uplink and the downlink, and/or is performed by using a shared channel on the downlink. A home base station and adjacent base stations each transmit a common pilot signal CPICH (Common Pilot Channel). A mobile station performs synchronization establishment, channel estimation, and the like by receiving the common pilot signal, and then performs data transmission/reception with the base station. Therefore, it is necessary to make it possible to receive a common pilot signal with good reception quality in the mobile station in order to provide good communication quality (for example, see Non-patent literature 2). Further, each base station broadcasts cell information including a cell ID at regular intervals by using a BCH (Broadcast Channel), and a mobile station identifies the base station based on the broadcasted cell information.
In base stations in existing mobile communication networks, the transmission power of a common pilot signal to be transmitted in each cell is set in a fixed manner. In contrast to this, as for a common pilot signal transmitted by a home base station in a home cell, a way of autonomously setting its transmission power and frequency by the home base station itself has been studied. Patent literature 1 (page 14, line 8 to page 15, line 21), for example, discloses such a method. Further, it has been also studied to develop a way of operation in which the home base station receives adjacent cell information including an adjacent cell ID, a used frequency, and a primary scrambling code used for encoding from an apparatus on an upper-layer network, measures received power or the like from adjacent cells based on this adjacent cell information, and sets an optimal frequency and transmission power value to be used in the home base station (see Non-patent literature 3). In the method like this, the home base station measures downlink received power RSSI (Received Signal Strength Indicator) from adjacent base stations and/or received power RSCP (Received Signal Code Power) of CPICH for each frequency, sets a frequency having the lowest measurement value for the home base station.
Further, in the W-CDMA system, five states consisting of Idle Mode, URA_PCH, CELL_PCH, CELL_FACH, and CELL_DCH are defined as the communication states of a mobile station (see Non-patent literature 4). A mobile station in a CELL_DCH state specifies a base station to which the mobile station belongs, and communicates with it by using a dedicated channel. Meanwhile, a mobile station in a state other than the CELL_DCH state does not specify the base station to which the mobile station belongs, and selects a base station every time the mobile station transmits control information and the like. In Release 7 of 3GPP, a new state “Enhanced CELL_FACH” was added as another state of a mobile station (see Non-patent literature 5). In Enhanced CELL_FACH, a mobile station in a CELL_FACH state uses HSDPA (High Speed Downlink Packet Access) in order to increase the communication speed. In HSDPA, H-RNTI (HS-DSCH Radio Network Temporary Identifier) is assigned to each mobile station. A control channel HS-SCCH (Shared Control Channel for HS-DSCH) is a channel used to notify control information to a mobile station, and is encoded by using H-RNTI to distinguish a destination mobile station. Specifically, a CRC value, which is added to HS-SCCH, is masked by H-RNTI of the destination mobile station. A mobile station receives HS-SCCH, and when it can be decoded by using the assigned H-RNTI (i.e., when the masked CRC value can be decoded by using its own H-RNTI), the mobile station determines that data destined for that mobile station is being transmitted. Further, an identifier “Common H-RNTI” is also defined to notify information to all mobile stations belonging to a cell. When HS-SCCH is encoded by using Common H-RNTI, all the mobile stations belonging to the cell can receive information included in HS-SCCH.